Project Summary/Abstract Restricting daily calorie intake is the primary dietary strategy currently used to treat obesity. However, this approach is not effective for all individuals. Further, adherence to daily caloric restriction (DCR) typically decreases over time and many individuals who lose weight with this strategy will regain. Thus, novel dietary interventions are needed to provide a broader range of treatment options. The objective of this proposal is to determine if intermittent fasting (IMF) is an effective dietary strategy for treatment of obesity. IMF is an alternative method of reducing energy intake (EI) which has gained lay popularity in recent years as a strategy for weight loss. Short-term studies suggest IMF produces 3-8% weight loss in individuals with overweight and obesity. However, few studies have compared IMF to DCR and no studies have evaluated whether IMF is an effective long-term (?1 year) weight loss strategy. Data from an 8-week pilot study comparing IMF to DCR suggests IMF is safe, tolerable, and produces similar short-term weight loss. After 6 months of unsupervised follow-up, changes in body composition tended to be more favorable with IMF and almost twice as many IMF participants maintained a ?5 kg weight loss. Thus, IMF may be a more effective dietary strategy for sustaining weight loss than DCR. However, longer-term data are needed and little is known about adherence to IMF or the impact of IMF on energy expenditure (EE). Short-term studies suggest adherence to IMF may be superior to DCR, however, energy intake (EI) was based on self-report which may have significant limitations. Pilot data suggests resting energy expenditure (REE) was preserved during IMF-induced weight loss, which could decrease risk for weight regain. However, the impact of IMF on other components of EE including physical activity (PA) is largely unexamined. The study design is a pragmatic 1-year randomized trial to compare weight loss generated by IMF vs DCR. The targeted weekly energy deficit will be equivalent: DCR will be prescribed a 30% daily energy restriction and IMF will be prescribed a modified fast (75% energy restriction) on 3 non-consecutive days per week. The hypothesis is that a weight loss program based on IMF as the primary dietary strategy will result in greater weight loss at 1 year compared to a program based on traditional DCR. Aim 1 will compare weight loss and changes in body composition and metabolic parameters induced by IMF and DCR at 1 year. Aim 2 will evaluate the impact of IMF (compared to DCR) on EI and dietary adherence. Aim 3 will evaluate the impact of IMF (compared to DCR) on components of EE and patterns of PA. Aim 4 will explore predictors of weight loss within both IMF and DCR. The approach is innovative as the targeted energy deficit will be matched between groups and EI will be assessed objectively (using doubly-labeled water) to provide an accurate comparison of adherence. In addition, both intervention groups will receive guidelines-based behavioral support to compare IMF to the current standard of care (DCR) in a more robust fashion. This study is significant as it could identify an effective alternative dietary strategy to help more people achieve and sustain weight loss. !